A new USB power delivery (PD) specification is under development to enable delivery of higher power over new USB cables and connectors. The intent for this technology is to create a universal power plug for laptops, tablets, and other devices that may require more than 5V. The USB PD specification defines a communication link between ports connected via a USB-PD cable and connectors. The communication is designed to be half-duplex and packet-based. The packets contain various information that enables the two ports to communicate and negotiate, including the voltage and the current that the source port will provide to the sink port. The ports can even negotiate to switch roles (Source to Sink and vice versa). The underlying communication in the USB PD specification is binary frequency shift keying (FSK). This communication happens independently from normal USB communications that go through the same cable but over different wires. The USB PD communication goes over the “Vbus” wire rather than the USB data wires.
Since legacy cables and even new PD cables have different current ratings, one piece of the new specification is the ability to detect the cable-type and thereby know the current rating of a cable. USB PD devices are not allowed to request or offer a voltage or current exceeding the ratings of the cable. Legacy cables (standard B, standard A, micro-A or micro-B) are limited to 1.5 A and 5V. PD micro cables (type A and B) are limited to 3 A. PD standard cables are limited to 5 A.
In an equivalent circuit for transmission in a USB PD system, ideally rTX=Z0 so that there are not any reflections from the cable during transmission. In some cases, the receiver may also use a value of rRX approximately equal to Z0 so that there are not reflections back onto the cable. In other cases, the receiver may have rRX set to a value much greater than Z0, in which case the line is effectively not terminated. In the latter case, if the cable length is approximately a quarter of a wavelength, the voltage seen at the input to the cable on the TX side is close to zero. This creates some challenges for a conventional USB PD system.